126 STUDY OF COMMON PLANTS. 



RELATIONSHIP. 



With the species already studied compare others of the 

 same genus, such as Eqwisetum hiemale, L., E. limosum, L., 

 etc. Do these species show the same general structure ? 

 Do they present the same differentiation into fertile and 

 sterile fronds ? 



Comparison with still other species of the single genus now 

 composing this family 1 shows that the Equisetinese. possess 

 very marked and characteristic features by which they are 

 distinguished from all other families of plants. At the 

 same time their close relationship with the ferns is evident 

 when their developmental history is followed out. If the 

 spores of the common horsetail are sown as directed above, 

 the development of the prothallium, including the forma- 

 tion of archegonia and antheridia, can be observed in detail 

 in the course of a few weeks, and affords a most instructive 

 study. 2 If this study is carried far enough to include the 

 formation of the embryo and growth of the young plant, it 

 is seen that the cycle of development is essentially identical 

 with that of the ferns. 



1 The horsetails are remnants of a family which once flourished luxuri- 

 antly, reaching its highest development in the Carboniferous period, when 

 there were several genera, including a number of tree-like species. 



2 Cf. Campbell, Male Prothallium of the Common Horsetail, Arner. 

 Nat., 1883, p. 10. 



